Why cant apt-get install the i386 version of libpcap-dev?

$ sudo apt-get install libpcap-dev:i386

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Package libpcap-dev:i386 is not available, but is referred to by another package.

This may mean that the package is missing, has been obsoleted, or

is only available from another source



E: Package 'libpcap-dev:i386' has no installation candidate

However, the package clearly exists: https://launchpad.net/ubuntu/trusty/i386/libpcap-dev. I'm on Ubuntu 14.04 x64.